Problem: In physics class, the girl to boy ratio is $6$ to $3$. If there are a total of $45$ students, how many girls are there?
Solution: A ratio of $6$ girls to $3$ boys means that a set of $9$ students will have $6$ girls and $3$ boys. A class of $45$ students has $5$ sets of $9$ students. Because we know that there are $6$ girls in each set of $9$ students, the class must have $5$ groups of $6$ girls each. There is a total of $30$ girls in physics class.